1. Field of the Invention
This invention relates to an image-wise exposure apparatus, wherein a photosensitive material is exposed image-wise by use of a mirror array device, a liquid crystal panel or the like, and more particularly to such an image-wise exposure apparatus, wherein a picture element shifting operation is carried out so that a high resolution image can be obtained on the photosensitive material.
Further this invention relates to a mirror array device and a liquid crystal panel for use in such an image-wise exposure apparatus.
2. Description of the Related Art
As described, for instance, in "O plus E" magazine, the October issue, 1994, pp. 90-94, there has been known a mirror array device comprising a plurality of small mirrors which are arranged in the form of a two-dimensional array, and a drive means for changing the orientations of the respective small mirrors independently from each other so that light impinging upon each of the small mirrors is reflected selectively toward one of two directions.
With such a mirror array device, an image can be projected onto a predetermined projection surface via the mirror array device while modulating the light impinging upon the projection surface for each of the small mirrors by controlling the drive means according to an image signal. When a photosensitive material is located on the projection surface, the photosensitive material can be exposed image-wise. Further the amount of light impinging upon the photosensitive material can be controlled for each of the small mirrors by, for instance, pulse width modulation of the "on" time of each small mirror (i.e., the time each smaller mirror is set in the orientation in which the light is caused to impinge upon the photosensitive material) within a frame time, whereby a gradation image can be formed on the photosensitive material.
In such a mirror array device, when large gaps exist between the small mirrors each of which forms one picture element, light travelling through the gaps is scattered by the base plate which supports the small mirrors or by mirror drive elements on the base plate. The scattered light can reach the plane on which an image is formed by the light reflected by the small mirrors and deteriorates the extinction ratio.
In order to avoid such a problem, it is ideal that the small mirrors are arranged very close to each other, that is, the mirror pitches are substantially equal to the mirror lengths in both the longitudinal direction and the transverse direction and the mirror aperture is almost 100%. Accordingly, in a mirror array device for use in an image-wise projection, there have been made attempts to make the mirror aperture as large as possible (e.g., not smaller than 85%) by forming each of the small mirrors to be sufficiently large to be close to adjacent mirrors.
Further, it has been proposed to utilize, when a photosensitive material is to be exposed image-wise by use of a spatial modulation device such as a liquid crystal panel, a so-called "picture element shifting" in order to increase the density of picture elements of the formed image, thereby obtaining a high resolution image. As described, for instance, in Japanese Unexamined Patent Publication No. 4(1992)-7542, with the picture element shifting, the optical relationship between the spatial modulation device and the photosensitive material is changed so that, after exposure dots have been formed on the photosensitive material by the light from the picture elements of the spatial modulation device (e.g., liquid crystal cells in the case of a liquid crystal panel), exposure dots may be further formed between adjacent exposure dots by the light from the picture elements of the spatial modulation device. The image-wise exposure is carried out each time the optical relationship is changed.
As for the image formed by the image-wise exposure, the aforesaid exposure dots constitute the picture elements of the formed image. However in this specification, in order to discriminate the picture elements of the formed image from the picture elements of the spatial modulation device, the picture elements of the formed image will be referred to as the exposure dots unless otherwise specified.
For example, when the spatial modulation device comprises (i).times.(j) number of picture elements arrayed along X and Y directions and the picture element shifting operation is carried out once along the X direction and once along the Y direction, 2i number of exposure dots and 2j number of exposure dots are recorded on the photosensitive material respectively along the X and Y directions thereof. That is, in this case, exposure dots which are the same in number as the exposure dots which can be recorded by one image-wise exposure using a spatial modulation device having 4.times.(i.times.j) number of picture elements can be recorded, whereby the resolution of the image formed on the photosensitive material can be increased.
The relationship between the spatial modulation device and the photosensitive material may be changed, for instance, by moving the spatial modulation device, the photosensitive material or an optical element between the spatial modulation device and the photosensitive material.
Generally in the case of a spatial modulation device such as a liquid crystal panel, it is difficult to integrate a large number of picture elements at a high density. However when the aforesaid picture element shifting is employed, a high-density image can be formed by image-wise exposure using a spatial modulation device having a relatively small number of picture elements arrayed at a relatively low density.
The picture element shifting operation can be carried out in an image-wise exposure apparatus comprising a mirror array device. However when the picture element shifting operation is carried out in an image-wise exposure apparatus comprising a conventional mirror array device, the sharpness of images formed on the photosensitive material deteriorates.
Further there has been known an image-wise exposure apparatus in which a gradation image displayed on a liquid crystal panel is recorded on a photosensitive material. Also in such an image-wise exposure apparatus, the sharpness of images formed on the photosensitive material deteriorates when the picture element shifting operation is carried out.